1. Field of the Invention
The present invention relates to a method of removing metal contaminants from organic dielectrics in general, and in particular, from polyimide which is typically used in multilevel thin-film interconnect structures for packaging applications.
2. Description of the Prior Art
Polyimide has gained increasing importance as a dielectric material in thin-film multilayer interconnect structures. Such structures are used in high-speed, high-density packaging, especially for main-frame computer applications. In these applications, multiple layers of interconnect metallizations are separated by alternating layers of polyimide whose function is to serve as the electrical isolation between the metal features.
More specifically, a typical substrate used for the above applications generally consists of a ceramic wafer with a spun-on film of polyimide on top of which fine line interconnections consisting of metallization features of Cu, Au, or Al are then patterned. However during the fabrication process, a typical step involves the deposition of reactive metals such as Ti:W, Cr, Ni, or Pd, on the ceramic-polyimide substrate. These metals serve as diffusion barriers, adhesion layers, or catalysts. They however have to be etched away at a later stage in the fabrication process, and although conventional etching techniques such as wet etching and plasma processing can etch away the bulk of these metals, these processes are unable to remove trace metal contaminants that adhere to the surface of the polyimide. Even trace quantities of these metals can severely limit the dielectric performance of the polyimide. It is therefore crucial to remove the contaminants and restore the dielectric performance of the polyimide to levels that are acceptable for packaging applications.
A typical method mentioned in the literature to address this type of problem is that of plasma etching. The technique of plasma etching serves to etch vias in polyimide for multilevel metallization systems. While the method can remove bulk polyimide, it is unsuited for problems that require only a minimal removal of polyimide and a selective removal of metal contaminants in it. For example, U.S. Pat. No. 4,357,203 (issued Nov. 2, 1982 to Joseph Zelez and assigned to RCA Corporation) discloses a process for removing the residual film remaining after oxygen plasma etching of polyimide by a second plasma etching utilizing a mixture of argon and hydrogen. While effective in removing residual polyimide under masked conditions, the '203 process would not serve to remove polyimide in the presence of gold metallizations, as it would sputter the gold, causing further contamination of the polyimide. If a mask were used to cover the gold in order to prevent sputtering, both the complexity and the cost of this process would be significantly increased.
Another patent, U.S. Pat. No. 4,445,966 (issued May 1, 1984 to Robert J. Carlson and Daniel W. Youngner and assigned to Honeywell Inc.) discusses a method of plasma etching of films containing chromium. The '966 patent discloses the use of fluorine containing compounds and deals strictly with the removal of chromium from silicon substrates. The surfaces of these substrates are entirely different from sensitive polyimide surfaces and therefore the '966 technique dies not teach or suggest a solution to the problem of removal of chromium or any other metal contaminants from polyimide.
Excimer laser bombardment has been used for the removal of non-bound or free-standing particles from solid surfaces. (A. C. Tam, W. Zapka, and W. Ziemlich, "Efficient laser cleaning of small particulates using pulsed laser irradiation synchronized with liquid-film deposition, "SPIE, 1598, 13-18, 1991.) The technique applied by these authors uses excimer laser bombardment of a surface in conjunction with a liquid jet such as that of water. This process differs from the present invention in that it involves a wet process (not a dry one), and requires a complex experimental set-up involving a specially designed pulsed liquid film deposition system. In addition, the process used by these authors specifically addresses the removal of airborne particles and not embedded particles as in the present invention. In a patent by Karl Asch, Joachim Keyser, Klaus Meissner, and Werner Zapka, issued Dec. 25, 1990 and assigned to International Business Machines Corporation (U.S. Pat. No. 4,980,536), airborne particulates of extremely small dimensions (100-1000 nanometers) were removed from the type of silicon membrane masks that are used primarily in electron beam lithography. The '536 technique uses a mask to selectively expose desired areas of the substrate. In addition, the '536 technique involves the removal of airborne particles (not embedded contaminants) and addresses an application that is quite specific to lithographic masks. This is quite different from the approach of the present invention that teaches a technique to selectively target a single material on a multi-component substrate, based upon the differences in ablation thresholds of each component, thereby excluding the need for a mask.
Excimer laser ablation of organic polymers, in particular that of polyimide has drawn tremendous interest for over a decade, resulting in numerous studies. (R. Srinivasan and W. J. Leigh "Ablative photodecomposition: action of far-ultraviolet (193 nm) laser radiation on poly(ethylene terephthalate) films," J. Am. Chem. Soc. 104, 6784-6785, 1982.) (J. E. Andrew, P. E. Dyer, D. Forster, and P. H. Key, "Direct etching of polymeric materials using a XeCl laser," Appl. Phys. Lett. 43, 717-719, 1983.) (R. Srinivasan and B. Braren, "Ablative photodecomposition of polymer films by pulsed far-ultraviolet (193 nm) laser radiation: dependence of etch depth on experimental conditions," J. Polym. Sci. 22, 2601-2609, 1984.) (G. Koren and J. T. C. Yeh "Emission spectra, surface quality, and mechanism of excimer laser etching of polyimide films," Appl. Phys. Lett. 44, 1112-1114, 1984.) (J. H. Brannon, J. R. Lankard, A. I. Baise, F. Burns, and J. Kaufman, "Excimer laser etching of polyimide," J. Appl. Phys. 58, 2036-2043, 1985.) (R. Srinivasan, B. Braren, and R. W. Dreyfus, "Ultraviolet laser ablation of polyimide films," J. Appl. Phys. 61, 372-376, 1987.) The technological importance of excimer laser ablation of polyimide has been realized through applications such as the production of via-holes, (F. Bachmann, "Large scale application for excimer-lasers: via-hole-drilling by photo-ablation," SPIE 1377, 18-29, 1990.) micropatterning of surfaces, (J. H. Brannon, "Micropatterning of surfaces by excimer laser projection," J. Vac. Sci. Technol. B 7, 1064-1071, 1989.) and patterned electroless plating. (H. Niino and A. Yabe, "Positively charged surface potential of polymer films after excimer laser ablation: application to selective-area electroless plating on the ablated films," Appl. Phys. Lett. 60, 2697-2699, 1992.) While demonstrated for applications requiring the bulk removal (several microns) of polyimide, there are no known efforts to use excimer laser ablation for surface cleaning of polyimide and the removal of metal contaminants from it. In yet another invention, U.S. Pat. No. 4,882,200, (issued to Liu and Grubb on Nov. 21, 1989 and assigned to General Electric Company), an excimer laser is employed to pattern electroless plating activator material from polymer and other substrates. In the ' 200 process, the activator is intentionally deposited on the substrate. Hence it never becomes embedded in it. This is in contrast to the present invention where the contaminants to be removed are embedded in and are part of the substrate. In addition, the '200 technique is not and cannot be used as claimed to selectively remove trace metal contaminants without selectively exposing the polymer. In the present invention, the surface of the contaminated organic dielectric is completely, not selectively, exposed.
It is therefore an object of this invention to selectively remove trace metal contaminants from an organic dielectric using excimer laser ablation in a maskless process not requiring selective exposure of the substrate. Yet another object is to provide process windows that will achieve this result with a minimal removal of the dielectric material. Another object is to leave pre-patterned thin-film metallization features on the surface of the organic dielectric unaffected and intact to enable further processing steps. Another object is to scale up this process for large area applications using, for example, step and repeat processing.